P1273: INVESTIGATING THE ONCOGENIC POTENTIAL OF STAT5BN642H IN NK CELLS
Authors: K. Klein, M. Randhawa, A. Witalisz-Siepracka, J. Trifinopoulos, P. Kudweis, V. M. Knab, D. Gotthardt, V. Sexl
Journal: HemaSphere (2022)

Background: The signal transducer and activator of transcription 5B (STAT5B) regulates development, survival and function of innate(-like) lymphocytes. STAT5B gain-of-function (GOF) mutations, including the most common STAT5BN642H mutation, are found in different hematopoietic malignancies. These include aggressive NK-cell malignancies and CD56+ T-cell large granular lymphocyte leukemia (T-LGLL). A previously established mouse model expresses human STAT5BN642H under the Vav1 promoter and thus in the entire hematopoietic system. These Vav1-driven STAT5BN642H transgenic mice develop severe CD8+ T-cell malignancies. In a transplantation setting devoid of CD8+ T cells, Vav1-driven STAT5BN642H also transforms NKT cells and promotes aggressive NKT-cell leukemia, resembling CD56+ T-LGLL. Aims: We aim to investigate whether STAT5BN642H also acts as an oncogenic driver in NK cells and establish an NK-cell leukemia model, which would enable the development of treatment options for highly aggressive NK-cell malignancies. Methods: While Vav1-driven STAT5BN642H caused a temporary expansion of NK cells, no NK-cell disease was observed with this model. To avoid competition with more potently transformed cell types, we generated a novel mouse model in which STAT5BN642H expression is restricted to the NK (NKp46-positive) cell lineage (N642HNK/NK mice). NK cells were characterized by flow cytometry in 8-12-week-old mice. N642HNK/NK and control mice were aged to investigate whether mutant STAT5B has an oncogenic potential in NK cells. Furthermore, NK-cell functionality was assessed in vitro upon IL-2 culture and in vivo with the subcutaneous NK cell-sensitive RMA-S tumor model to study effects of enhanced STAT5B signaling on NK-cell activity. Results: STAT5BN642H expression in NKp46+ cells resulted in an increased number of NK cells in young and aged mice but failed to cause signs of an aggressive NK-cell malignancy up to an age of 10-14 months. Ageing experiments will be continued until an age of 15-18 months to investigate an oncogenic potential of mutant STAT5B in NK cells at a later timepoint. Overexpression of non-mutant STAT5B or STAT5BN642H decreased the expansion capacity of NK cells in vitro, associated with a “senescence-like” state and reduced cytotoxic capacity. On the contrary, overexpression of non-mutant STAT5B or STAT5BN642H in NKp46+ cells enhanced tumor surveillance of RMA-S tumors in vivo. Summary/Conclusion: Our current data suggest that STAT5BN642H on its own is not able to transform NK cells, in contrast to other cell types. Additional alterations might be required to allow for NK-cell transformation by STAT5BN642H and to cause a symptomatic disease. Enhanced STAT5B signaling has opposing effect on NK-cell functionality in vitro compared to in vivo. Exploring the underlying mechanisms will contribute to a better understanding of the complex regulation of NK-cell activity. This might have implications for immunotherapeutic approaches using cytokines that activate STAT5 to expand and stimulate NK cells.